1. Field of the Invention
The present invention relates generally to optical fiber devices and methods, and in particular to improved systems and methods for cascaded Raman lasing at high power levels.
2. Background Art
Fiber lasers and amplifiers are typically based on optical fibers that are doped with laser-active rare earth ions, such as ytterbium (Yb), erbium (Er), neodymium (Nd), and the like. Stimulated Raman scattering in optical fibers is a useful effect that can be employed in order to provide nonlinear gain at wavelength regions in which these fibers do not operate. Stimulated Raman scattering occurs when a laser beam propagates through a Raman-active fiber, resulting in a predictable increase in wavelength, known as the “Stokes shift.” By providing a series of wavelength-specific reflector gratings at the input and output ends of a length of a Raman-active fiber, it is possible to create a cascaded series of Stokes shifts in order to convert a starting wavelength to a selected target wavelength.
FIG. 1 is a diagram of one exemplary system 20 according to the prior art, in which stimulated Raman scattering is used to generate a high-power output for pumping an erbium-doped fiber amplifier (EDFA). As illustrated, the system 20 comprises two stages: a monolithic Yb-fiber laser 40 and a cascaded Raman resonator (CRR) 60.
In laser 40, the active medium is provided by a length of a double-clad Yb-doped fiber 42 operating in the region of 1000 nm to 1200 nm. A high reflector grating 44 is provided at the input end of fiber 42, and an output coupler grating 46 is provided at the output end of fiber 42. In the present example, gratings 44 and 46 are written into separate segments of passive fiber that are fused to fiber 42. It would also be possible to write gratings 44 and 46 directly into the input and output ends of fiber 42.
High reflector 44, output coupler 46, and the fiber 42, together function as a laser cavity 48. Pumping energy is provided to fiber 42 by a plurality of pump diodes 50, which are coupled to fiber 42 by means of a tapered fiber bundle (TFB) 52. In the present example, laser 40 provides as an output single-mode radiation at a wavelength of 1117 nm.
The laser output is launched as an input into CRR 60. CRR 60 comprises a Raman-active fiber 62, including a first plurality of high reflector gratings 64 provided at its input end, and a second plurality of high reflector gratings 66 provided at its output end. Also provided at the output end of the Raman fiber 62 is an output coupler grating 68. In the present example, input gratings 64 and output gratings 66 are written into separate segments of passive fiber that are fused to fiber 62. It would also be possible to write gratings 64 and 66 directly into the input and output ends of fiber 62.
Input high reflectors 64, output high reflectors 66, output coupler 68, and Raman fiber 62 provide a nested series of Raman cavities 70, which create a cascaded series of Stokes shifts over a broad range, increasing the 1117 nm input wavelength to a 1480 nm target wavelength in a series of steps. Output coupler 68 provides a system output 72 at a target wavelength of 1480 nm, which can then be used to pump a high-power erbium-doped fiber amplifier (EDFA) in the fundamental mode.
System 20 may be used for other applications requiring output wavelength other than 1480 nm and may be configured to generate output wavelength in only a single step.
While FIG. 1 shows cascaded Raman resonator constructed using gratings 64, 66 and 68, similar resonators are well known using other wavelength selective elements such as fused-fiber couplers and thin-film filters and other architectures such as WDM loop mirrors. In addition, linear, unidirectional ring or bidirectional ring cavity geometries can be considered. Furthermore, FIG. 1 shows the cascaded Raman resonator configured to operate as a laser, but it equally well could be configured to operate as an amplifier by leaving off the final set of gratings and instead injecting a signal at that wavelength. Similar to the configuration shown in FIG. 1, these additional configurations will increase the 1117 nm input wavelength to a 1480 nm target wavelength in a series of steps.
The prior art system 20 suffers from known limitations. For example, one issue arises due to the fact that multiple reflectors at various wavelengths and positions in the system 20 combine to create coupled cavities. For example, it will be seen that there are three reflectors at the laser wavelength of 1117 nm, i.e., reflectors 44 and 46, and the leftmost member of output reflector group 66. In general this does not pose a problem for relatively low power systems (e.g., 5 W output at 1480 nm), but does pose a problem for high power systems. Recently, investigations have been undertaken with respect to power scaling of Raman fiber lasers, and power levels as high as 41 W have been demonstrated from a CRR. A similar situation arises in cascaded Raman resonators constructed using other well-known architectures, such as with WDM loop mirrors.
While high power has been demonstrated from such a system, the coupled cavity nature of the setup in FIG. 1 has serious implications on long-term reliable operation. In particular, the coupled cavity can cause the system to become unstable and generate pulses with sufficiently high peak-power to damage components. The laser high reflector 44 in particular has been found to be a weak link in the system, presumably due to the high power that propagates through it, and has been observed to fail under various conditions including, for example, using the system 20 to pump an erbium-doped fiber amplifier, or like device. In addition, it is possible for light from intermediate Stokes orders generated in the Raman laser to propagate back into the Yb amplifier and back to the pump diodes, causing them to fail. Furthermore, light at the first Stokes shift is within the gain bandwidth of Yb and is amplified before hitting the diodes. It will be apparent that this is also detrimental.